1. Field of the Invention
The invention pertains to a method for the compression of digital images and, more especially, to a method for the compression of digital images intended to be televised. It also pertains to a device for the application of a method of this type.
2. Description of the Prior Art
Television images comprise pixels which are evenly distributed in the plane of the image, pixels to which one or more radiometric qualities, called components, are assigned.
These components may be, for example, the measurements of the radiometric characteristics R.sub.i, V.sub.i, B.sub.i of each pixel in the case of three channels, red, green and blue, for a color image or radiometric measurements of each pixel in the case of only one channel for black and white images.
The digitalization of an image for its processing or transmission conventionally consists in the encoding of each pixel in a number B of bits. The total digital volume representative of an image is equal to N.times.M.times.B bits, M being the number of image lines and N the number of pixels per line.
In certain applications it is absolutely necessary to use small-sized memories. It is therefore necessary to reduce, as far as possible, the volume of the digital information which has to be stored in these memories.
For this, techniques such as differential encoding or orthogonal transformations or statistical encoding have been experimented with, but have not given very satisfactory results. For these techniques do not make it possible to obtain a high compression rate without the deterioration of the images after they are restored.
Besides, it is known that, to obtain an image compression rate, the pixels are assembled, according to a predetermined criterion, into classes, the number of classes being smaller than the number of combinations of encoding bits of a pixel in order to obtain a comression.
It is also known that, to obtain a compression, the image is fictitiously cut up into windows and that the number of classes are fixed on an a priori basis for each window. Each class is identified by a predetermined value. The application of an assembling criterion to each pixel makes it possible to assign this pixel to one of these classes. When all the pixels of a window have been classified, it is usual to calculate a vector, for example, by calculating the center of gravity for each class so as to identify each class by a new vector in order to improve the precision.
This type of image analysis remains, all the same, unsuited to certain applications. For window-by-window processing cannot be warranted for all applications, especially for applications in which the images have few shades. Besides such processing requires previous analysis of the image in order to determine each vector which will make it possible to identify a class. This preliminary study can make it possible to determine the initial radiometric values used to create each class.